The present invention relates to a toroidal continuously variable transmission (CVT) useable for automobiles.
Toroidal CVTs include generally toroid curved surface input and output disks coaxially arranged in spaced and opposed relation, and a plurality of power rollers disposed within a toroidal cavity formed between the input and output disks. A traction oil film is disposed between traction surfaces of the input and output disks and traction surfaces of the power rollers. The power rollers rotate to transmit rotation of the input disk to the output disk via the traction surfaces and the traction oil film. During the transmission of rotation, the power rollers swing relative to the input and output disks so that the mutually contacted portions between the power rollers and the input and output disks are displaced to thereby continuously change the gear ratio.
Japanese Patent Application First Publication No. 11-148542 discloses the toroidal CVT in which traction surfaces of power rollers have an arithmetical mean roughness Ra of 0.05 xcexcm or less. The earlier technique aims to prevent the traction oil film between the traction surfaces of the power rollers and input and output disks from being less formed or reduced in thickness due to heat and shear stress which are caused by spin of the traction surfaces, to thereby restrain deterioration of durability of the toroidal CVT.
The traction surfaces of the power rollers which have the surface roughness of 0.05 Ra or less as described in the earlier technique, are relatively smooth. It will be appreciated that the traction oil film formed between the traction surfaces of the disks and power rollers has an excessive thickness under condition that rolling speed of the disks and power rollers is high. The excessively thickened traction oil film causes reduction of traction coefficient of the toroidal CVT.
Generally, it is known that in the toroidal CVTs, the traction surface of each of the power rollers is in contact, at substantially the same circumferential portion, with the traction surfaces of the disks, regardless the gear ratio. On the other hand, the traction surface of each of the disks is in contact, at different circumferential portions, with the traction surfaces of the power rollers. This is because the contact portions of the traction surfaces of the disks contacted with the traction surfaces of the power rollers is displaced between the increased-diameter side and reduced-diameter side of the disks depending on the gear ratio. The circumferential speeds of the disks, therefore, are variable depending on the degree of displacement of the contact portions of the traction surfaces of the disks contacted with the traction surfaces of the power rollers, even if the rotation number of the input disk is the same.
In addition, it is also known that change in thickness of an oil film is less affected by load (torque in transmissions) but the change is considerably affected by circumferential speed and temperature. This is appreciated from the following formula of Hamrock-Dowson in calculation of minimum oil film thickness Hmin.
Hmim=3.63U0.68G0.49Wxe2x88x920.073 {1xe2x88x92exp(xe2x88x920.68k)}
where
U=xcex7ou/ER
G=xcex1E
W=w/ER2 
u=(u1+u2)/2
xe2x80x83where
U is a parameter of speed, G is a parameter of material, W is a parameter of load, k is a parameter of ellipse, xcex7o is an oil viscosity under atmospheric pressure, u is a circumferential speed, E is an equivalent elastic coefficient, R is an equivalent radius of curvature, xcex1 is a pressure coefficient in oil viscosity, w is a load, and u1 and u2 are circumferential speeds of two rolling elements, respectively.
In the toroidal CVT of the earlier technique as described above, the surface roughness of the traction surface of each disk is not disclosed. Assuming that the surface roughness of the traction surface of the disk is set at a uniform roughness value for a relatively smooth surface in order to assure durability of the disk, the thickness of the traction oil film will be inadequately increased in HIGH speed range where the circumferential speed is high and the traction oil film is likely to be formed. Namely, in the HIGH speed range, the input disk is operated on the increased-diameter side of and the output disk is operated on the reduced-diameter side. Further, if the oil film thickness is inadequately increased, a shear rate will be lowered to thereby undesirably reduce a traction coefficient. A shearing force generated by the traction oil is simply expressed by the formula: (viscosity)xc3x97(shear rate). The viscosity of the traction oil decreases at high temperature. Therefore, if the temperature of the traction oil is high, the traction coefficient will be reduced.
There is a demand to solve the above-described problems in the earlier technique and maintain high traction coefficient during the operation of the toroidal CVT.
Meanwhile, the loading force applied to the power roller by the disks may be increased in order to suppress the formation of an excessively thickened traction oil film therebetween. In this case, reduction of the size of the disks and power roller will cause great difficulty due to limited material strength. In addition, loss in power transmission which is generated at bearings within the toroidal CVT, will be increased, resulting in lowering fuel economy.
Further, in the toroidal CVT, the traction surface of the power roller includes a so-called contact ellipse, i.e., an elliptical contact area where the traction surface of the power roller is in contact with that of each disk. During the operation of the toroidal CVT, high contact surface pressure is exerted onto a central portion of the contact ellipse. This causes fatigue at the central portion of the contact ellipse so that peeling or abrasion will occur there. On the other hand, low contact surface pressure is exerted on both end portions of the contact ellipse which are opposed to each other in the swing direction of the power roller. However, slippage occurs due to spin at the both end portions of the contact ellipse. This will cause wear at both end portions of the contact ellipse when the thin traction oil film is formed during the starting operation of the toroidal CVT. There is a demand to solve the above-described problems and improve both the durability and the traction coefficient in the toroidal CVT.
An object of the present invention is to provide a toroidal CVT which includes input and output disks and power rollers interposed therebetween, capable of restraining a thickness of the traction oil film formed between the disks and the power rollers from being excessively increased, thereby maintaining high traction coefficient, and achieving improvement in durability.
According to one aspect of the present invention, there is provided a toroidal continuously variable transmission, comprising:
an input disk;
an output disk, the input and output disks having a common first rotation axis about which the input and output disks are rotatable; and
a power roller swingably interposed between the input and output disks and having a second rotation axis intersecting the first rotation axis, about which the power roller is rotatable,
the input and output disks and the power roller having traction surfaces cooperating with each other to transmit rotation between the input and output disks and the power roller,
at least one of the input and output disks and the power roller having a surface roughness on the traction surface thereof which varies in a direction of the rotation axis thereof.